Modeling the effect of cage drag on particle residence time within fish farms in the Bay of Fundy

Abstract

Aquaculture farm cages have the ability to interact with local circulation due to the drag caused by the cages. We examined how the drag influences the residence time of particles within fish farms in the southwest Isles region of New Brunswick, Canada, in the Bay of Fundy using a high-resolution hydrodynamic model. To accomplish this, we parameterized the cage drag in the model and modeled flow structures at multiple spatial scales, ranging from several meters within the cages, to tens of kilometers in the adjacent open ocean. We used models with and without cage drag to demonstrate how residence time was influenced by the imposition of the cage infrastructure. Our examination indicated that the drag produced by cages is able to significantly change the residence time of particles. The magnitude of the change is strongly sensitive to the timing of tides, tidal speeds and specific locations of farms. Our results suggest that the flushing properties of the wastes from aquaculture activities—for example, feed and subsequently fecal material—are strongly related to flow properties and their interactions with cages. These results emphasize that the design of fish farms should explicitly account for the influence of physical infrastructure (i.e. cages) on depositional processes in order to try and minimize environmental effects.